1. Field of the Invention
The present invention relates to an interface between a network for transmitting a bandwidth-compressed signal and a network for transmitting a signal which is not bandwidth-compressed. Particularly, in an interface between a switching node of a core network, which is connected to a communication network and use to decode the bandwidth-compressed audio signal transmitted through a radio access network and a switching node of the radio access network, which is connected to the communication to transmit a bandwidth-compressed audio signal, the present invention relates to an insertion control of a transcoder, which is provided in the switching node of the core network. Incidentally, the switching node of the radio access network is referred to a RNC (Radio Network Controller) and the switching node of the core network having the transcoder may be a MSC (Mobile Switching Center) or a MGS (Media Gateway Server), which has a gateway switching function. Since these switching nodes, that is, the MSC and the MGS, are the same in an insertion control operation of the transcoder except functional roles thereof in the core network, the MSC will be described as a typical example of the switching node of the core network, which includes the transcoder.
2. Description of the Related Art
A new mobile communication system has been proposed, which is constructed with a core network provided as a fixed network, for performing a position control, a call control and a service control and at least one radio access network for controlling and terminating a radio technology and has an interface between the RNC of the radio access network and the MSC of the core network, defined as user interface Iu. The Iu interface is defined as having a hierarchical structure constructed with a control layer in which the switching nodes exchange control signals and a user layer in which users exchange transmitting signals and control signals. The user layer is constructed with a portion for transmitting a transmitting signal such as audio signal or data as AAL2 cell of an ATM (Asynchronous Transfer Mode) and a portion including a control signal for performing a negotiation between users. When a communication is performed between a calling terminal and a called terminal, audio signal or data signal is transmitted and received as a ATM packet signal after the negotiation is performed between the terminals by using the user layer. This communication system is shown in FIG. 8.
Since there are communications between the core network and the fixed cable telephone network and between the core network and radio access networks, audio signal is transmitted in the form of a 64 kbps PCM signal within the core network. This is because the audio signal is standardized such that it is transmitted and received as the 64 kbps PCM signal in view of interfaces between the cable telephone network and other networks. On the other hand, in the mobile communication system, in order to effectively utilize radio frequency, which is finite resource, a transmitting audio signal is bandwidth-compressed. A CODEC for bandwidth-compressing the audio signal is provided in each terminal so that the audio signal is transmitted as a signal having narrow band-width, such as 9.6 kbps, in the radio access network. Therefore, a transcoder (TC) is provided in each switching node MSC of the core network, for converting the bandwidth-compressed audio signal into the 64 kbps PCM signal and transmitting the latter in the core network and compression-coding the 64 kbps PCM signal and transmitting the latter to the radio access network so that the conversion between the bandwidth-compressed audio signal and the 64 kbps PCM signal is performed. FIG. 9 shows the system in which the bandwidth-compressed audio signal is transmitted by converting it into the 64 kbps PCM signal by using the transcoder TC.
Assuming that a receiving side is a radio terminal utilizing the same audio signal compression coding system as that used in a transmitting side, a CODEC is inserted into each of at least four locations in a route from a transmitting side to a receiving side. That is, a CODEC in a transmitting terminal, a transcoder of a switching node MSC on the transmitting side, a transcoder of a switching node MSC on the receiving side and a CODEC in the receiving terminal are necessary to code and decode the signal. Therefore, distortion of transmitting signal is accumulated due to the coding and decoding operations of these CODECs, resulting in a degraded audio signal quality. Further, if the transcoders are provided in all of channels of the switching node MSCs, the cost for installing the transcoders is increased and a signal delay occurs due to the signal conversions.
In a case where the same audio signal compression coding system is employed in the respective mobile communication networks, each of the transmitting and receiving terminals has to equip with a CODEC capable of performing the same processing. Therefore, there is no need of converting the audio signal into the 64 kbps PCM signal by inserting the CODEC into each switching node MSC. It is clear that there is no problem if the audio signal is transmitted as a bandwidth-compressed, narrow band signal within the core network without using a transcoder. In such case, it is possible to reduce the cost of installation of the transcoders and to improve the quality of audio signal since the number of the coding and decoding steps is reduced. Therefore, in the mobile communication networks to which the same compression coding system can be applied, a system in which a bandwidth-compressed audio signal is transmitted as it is, without inserting transcoders into the switching nodes in the core network. The latter system is referred to as Transcoder-Free-Operation (TrFO) system.
The Transcoder-Free-Operation system is shown in FIG. 10.
When a switching node MSC is controlling a communication between a transmitting terminal and a receiving terminal by by-passing a transcoder thereof according to the Transcoder-Free-Operation system, there may be a case where a tone signal in an audio frequency band is to be sent to either one or both of the transmitting terminal and the receiving terminal. For example, there is a case where a tone signal indicative of another call is to be inserted into the transmitting and receiving terminals. Further, when a transmitting terminal is of the prepaid system and a prepaid amount is substantially spent, it is necessary to insert a tone signal into the transmitting terminal.
A tone signal generator for generating such tone signal is provided in the switching node MSC in the core network and operates to insert the tone signal into at least one of the communicating terminals. The tone signal generated by the tone signal generator is a 64 kbps PCM signal. Therefore, when the tone signal is inserted into the switching node RNC in which a bandwidth-compressed audio signal communication is performed, it is necessary to convert the 64 kbps PCM audio signal into a bandwidth-compressed, a narrow band audio signal by a transcoder.
In such situation, when the switching node MSC of the core network inserts the transcoder into a call, which is performed through the Transcoder-Free-Operation connection, it is necessary to re-initialize the transcoder between the switching node MSC of the core network and the switching node RNC of the radio access network. This is because the switching node MSC of the core network does not recognize a parameter for the bandwidth compression coding in the call through the Transcoder-Free-Operation connection.
The new mobile communication system, it has been proposed to use an AMR (Adaptive Multi Rate Codec) as the compression coding system. The AMR can dynamically determine the conversion rate in a range from 4.75 kbps to 12.2 kbps, for example. Further, a service class is assigned according to a tolerable error rate. The assignment of the coding frame of the AMR is performed by transmitting a RFCI (Rab sub-Flow Combination Identifier) information, which assigns Service Data Unit (SDU) size of a table describing conversion types between users, by using the user layer and performing a mutual negotiation between a transmitting terminal and a receiving terminal.
As mentioned, in the Transcoder-Free-Operation using the AMR coding system, the setting information of the compression coding is determined by the users by using the user layer of the user interface between the switching node MSC of the core network and the switching node RNC of the radio access network. Therefore, when the switching node MSC of the core network tries to insert the transcoder into a call under communication through the Transcoder-Free-Operation connection, the switching node MSC does not recognize its RFCI information as mentioned previously. Therefore, in order to insert the transcoder into the call under communication through the Transcoder-Free-Operation connection, it is necessary to execute the initializing procedure between the transcoder to be inserted and the switching node RNC of the radio access network. When the re-initializing procedure is executed during the communication, it is necessary to execute a plurality of sequences such as resetting of a Radio Access Bearer between the switching node RNC and the switching node MSC and re-setting of a Radio Access Bearer between the terminal MS and the switching node RNC, etc. Therefore, if the insertion of the transcoder to the call through Transcoder-Free-Operation connection by performing the re-initializing procedures, there is a risk of such as delay of connection and instantaneous cut-off of sound, etc.